The Tn9 transposon - a genetic element which carries a gene for chloramphenicol-resistance flanked on both sides by IS1 (an insertion sequence) - possesses two remarkable features. (1) It is able to insert itself as a unit into a variety of unrelated chromosomes and at many different sites. (2) It can generate deletions of portions of itself and/or of adjacent chromosomal DNA. Satisfactory understanding of the genetic parameters of the latter activity has yet to be achieved. Part of the problem is that Tn9 can be so "plastic" that standard genetic crosses frequently do not yield expected progeny. BIBLIOGRAPHIC REFERENCE: Rosner, J. L. and Gottesman, M. M.: Transposition and deletion of Tn9. A transposable element carrying the gene for chloramphenicol resistance. In Bukhari, A., Shaprio, J. and Adhya, S. (Eds.): DNA Insertion Elements, Plasmids, and Episomes. New York, Cold Spring Harbor, 1977 (in press).